An organic electroluminescence (“electroluminescence” will be referred to as “EL”, hereinafter) device is a spontaneous light emitting device which utilizes the principle that a fluorescent substance emits light by energy of recombination of holes injected from an anode and electrons injected from a cathode when an electric field is applied. Since an organic EL device of the laminate type driven under a low electric voltage was reported by C. W. Tang of Eastman Kodak Company (C. W. Tang and S. A. Vanslyke, Applied Physics Letters, Volume 51, Pages 913, 1987), many studies have been conducted on organic EL devices using organic materials as the constituting materials. Tang et al. used a laminate structure using tris(8-quinolinolato)aluminum for the light emitting layer and a triphenyldiamine derivative for the hole transport layer. Advantages of the laminate structure are that the efficiency of hole injection into the light emitting layer can be increased, that the efficiency of forming excitons by recombination can be increased by blocking electrons injected from the cathode, and that excitons formed can be enclosed within the light emitting layer. As the structure of the organic EL device, a two-layered structure having a hole transport (injection) layer and an electron transport and light emitting layer and a three-layered structure having a hole transport (injection) layer, a light emitting layer and an electron transport (injection) layer are well known. To increase the efficiency of recombination of injected holes and electrons in the devices of the laminate type, the structure of the device and the process for forming the device have been studied.
As the light emitting material of the organic EL device, chelate complexes such as tris(8-quinolinolato)aluminum, coumarine derivatives, tetraphenylbutadiene derivatives, distyrylarylene derivatives and oxadiazole derivatives are known. It is reported that light in the visible region ranging from blue light to red light can be obtained by using these light emitting materials, and development of a device exhibiting color images is expected (For example, Patent Reference 1, Patent Reference 2 and Patent Reference 3).
It is recently proposed that an organic phosphorescent material other than a fluorescent material is used in the light emitting layer of an organic EL device (for example, Non-Patent Reference 1 and Non-Patent Reference 2). A great efficiency of light emission is achieved, as described above, by utilizing an organic phosphorescent material excited to the singlet state and the triplet state in the light emitting layer of an organic EL device. It is considered that singlet and triplet of the excited state are formed in relative amounts of 1:3 due to the difference in the multiplicity of spin when electrons and holes are recombined in an organic EL device. Therefore, it is expected that an efficiency of light emission 3 to 4 times as great as that of a device utilizing fluorescence alone can be achieved by utilizing a material emitting phosphorescent light.
In the organic EL devices described above, a construction formed by successively laminating layers such as an anode, a hole transport layer, an organic light emitting layer, an electron transport layer (a hole blocking layer), an electron transport layer and a cathode is used so that the excited state of the triplet or the excitons of the triplet do not disappear, and a host compound and a phosphorescent compound are used for the organic light emitting layer (for example, Patent Reference 4 and Patent Reference 5). In these patent references, 4,4-N,N-dicarbazole-biphenyl is used as the host compound. This compound has a glass transition temperature of 110° C. or lower, and the symmetry is excessively excellent since the skeleton structure of biphenyl is bonded to the nitrogen atom on the central line of the skeleton structure of carbazole molecule. Therefore, problems arise in that this compound is easily crystallized and that short circuit takes place and defect pixels are formed in the test of heat resistance of the device.
It is also found that, when the compound is vapor deposited, growth of crystals takes place at portions where foreign substances or protrusions are present, and defects are formed at an early time before the test of heat resistance and continue to grow with time. Carbazole derivatives having the three-fold symmetry are also used as the host compound. However, since these compounds have structures exhibiting an excellent symmetry with respect to the nitrogen atom on the central line of the skeleton structure of carbazole in the molecule, it is inevitable that growth of crystals takes place in the vapor deposition at portions where foreign substances or protrusions are present, and defects are formed at an early time before the test of heat resistance and continue to grow with time.
Devices having a fluorescent benzofuran compound or a fluorescent dibenzofuran compound are disclosed (Patent Reference 6). However, no specific descriptions are found on compounds having a substituent at the 7-position of benzofuran or at the 4-position or the 6-position of dibenzofuran, or no descriptions are found on exhibition of a unique property.
A compound having a 4-biphenylindol structure at the 4-position of benzofuran as the substituent is shown as an example of the host compound in a phosphorescent device, and an example using the compound as the host for an iridium complex which is a phosphorescent light emitting material emitting bluish green light is shown (Patent Reference 7). However, since substituents are present at all positions of the benzene ring substituted at the 4-position of benzofuran, the compound is unstable due to the great steric hindrance, and there is the possibility that the obtained EL device has a short life. No compounds having bonding at the 4-position of benzothiophene are clearly disclosed.
A compound having a skeleton structure of benzothiophene in which the skeleton structure of anthracene is essential is disclosed (Patent Reference 8). However, it is considered that application of this compound to a phosphorescence device is difficult since this compound has the skeleton structure of anthracene having a narrow triplet energy gap. A furan-based compound in which the skeleton structure of anthracene is essential is disclosed (Patent Reference 9). However, no descriptions are found on compounds having a substituent at the 7-position of benzofuran or at the 4-position or 6-position of dibenzofuran, or no excellent properties of these compounds are found. A benzofuran compound bonded to the skeleton structure of pyrene is disclosed (Patent Reference 10). However, it is considered that the application of the compound to the organic EL device of the phosphorescent type is difficult due to the narrow triplet state energy gap of the skeleton structure of pyrene. No examples using the compound are found.
In Patent Reference 11, a dibenzofuran compound having the skeleton structure of benzotriazole is described. However, it is considered that that the application of the compound to the organic EL device of the phosphorescent type is difficult due to the narrow triplet state energy gap. No examples using the compound are found.
[Patent Reference 1] Japanese Patent Application Laid-Open No. Heisei 8 (1996)-239655
[Patent Reference 2] Japanese Patent Application Laid-Open No. Heisei 7 (1995)-138561
[Patent Reference 3] Japanese Patent Application Laid-Open No. Heisei 3 (1991)-200289
[Patent Reference 4] U.S. Pat. No. 6,097,147
[Patent Reference 5] International Patent Publication No. WO 2001/41512
[Patent Reference 6] Japanese Patent Application Laid-Open No. Heisei 5 (1993)-109485
[Patent Reference 7] Japanese Patent Application Laid-Open No. 2004-214050
[Patent Reference 8] Japanese Patent Application Laid-Open No. 2004-002351
[Patent Reference 9] Japanese Patent Application Laid-Open No. 2005-047868
[Patent Reference 10] International Patent Publication No. WO 2004/096945
[Patent Reference 11] International Patent Publication No. WO 2005/054212
[Non-Patent Reference 1] D. F. O′Brien, M. A. Baldo et al., “Improved energy transfer in electrophosphorescent devices”, Vol. 74, No. 3, pp 442 to 444, Jan. 18, 1999
[Non-Patent Reference 2] M. A. Baldo et al., “Very high-efficiency green organic light-emitting devices based on electrophosphorescence”, Applied Physics letters, Vol. 75, No. 1, pp 4-6, Jul. 5, 1999